U.S. patent application number 10/293834 was filed with the patent office on 2004-03-11 for method and apparatus for high resolution tracking via mono-pulse beam-forming in a communication system.
Invention is credited to Brothers, Louis R. JR., Cangeme, John, Flaig, Alexander, Macmullan, Samuel J., Poor, H. Vincent, Rao, Tandhoni S., Schwartz, Stuart C., Upadhyay, Triveni N..
Application Number | 20040046695 10/293834 |
Document ID | / |
Family ID | 31996872 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040046695 |
Kind Code |
A1 |
Brothers, Louis R. JR. ; et
al. |
March 11, 2004 |
Method and apparatus for high resolution tracking via mono-pulse
beam-forming in a communication system
Abstract
Method and apparatus for high resolution tracking via mono-pulse
beam-forming in a communication system in which the capacity and
range of mobile or fixed wireless communication base stations are
improved by implementing a single or multiple antenna beam per
signal path. Adaptive beam-forming based on up-link direction-of
arrival estimation can be performed without using the
above-mentioned computationally intensive techniques.
Inventors: |
Brothers, Louis R. JR.;
(Dorchester, MA) ; Cangeme, John; (Billerica,
MA) ; Flaig, Alexander; (Concord, MA) ;
Macmullan, Samuel J.; (Carlisle, MA) ; Poor, H.
Vincent; (Princeton, NJ) ; Rao, Tandhoni S.;
(Ashland, MA) ; Schwartz, Stuart C.; (Princeton,
NJ) ; Upadhyay, Triveni N.; (Concord, MA) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
31996872 |
Appl. No.: |
10/293834 |
Filed: |
November 13, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60331423 |
Nov 15, 2001 |
|
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Current U.S.
Class: |
342/427 |
Current CPC
Class: |
G01S 3/32 20130101; H04B
7/0617 20130101; H01Q 25/02 20130101; H04B 7/086 20130101 |
Class at
Publication: |
342/427 |
International
Class: |
G01S 005/02 |
Claims
What is claimed:
1. The invention substantially as shown and described.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to base station equipment for
receiving and transmitting one or more signals from one or more
users, in which the signals may arrive at the equipment along a
multiplicity of paths, from possibly different directions, and with
possibly different delays.
[0002] A major concern for providers of wireless communications
services is system coverage and capacity. Future systems promise
data rates and an aggregate capacity significantly higher than
current systems. However, with conventional base stations, the
maximum link closure range will be decreased substantially for
users operating at higher data rates. As a result, the promised
data rates and aggregate capacity can only be supported in a small
region close to the base station.
[0003] Smart antenna systems have been discussed in the literature
as a means of increasing capacity and coverage over and above that
which can be provided with simple omni-directional antennas. They
achieve this through spatio-temporal correlation of desired signals
and co-channel interference within a cell. Interference suppression
is implemented by forming narrow radiation patterns, forming
radiation nulls on significant interference points or a combination
of the two. Smart antennas are implemented in several forms;
switched-beam, Direction-of-Arrival (DOA) or Optimum Combining
derived adaptive-beams. Some systems are analog where the beam is
formed in an RF manifold such as a Butler matrix but the most
flexible are those that are digitally formed.
[0004] Switched beam systems such as the one described in U.S. Pat.
No. 6,218,987 entitled "Radio Antenna System", form several fixed
beams in an RF Butler matrix with the ability to simultaneously
broadcast a common channel with a high gain wide beam. A similar
fixed beam system is described in U.S. Pat. No. 6,181,276 entitled
"Sector Shaping Transition System and Method" where a combination
of a set of fixed beams can be coherently combined, by analog
means, to form another beam that is better adapted to the area
loading of the cell. These switched-beam systems do not take
advantage of the maximum gain offered by the full aperture. As a
mobile moves through a cell it will suffer beam-width modulation as
it travels between the peaks of the several fixed beams.
Furthermore, switched-beam approaches simply further sub-divide a
cell into sub-sectors. Unfortunately, this method requires handoff
between the sub-sectors just as with a standard 3-sector system.
These handovers require valuable resources and ultimately reduce
the capacity of the network.
[0005] In the recent papers entitled "A comparison of Tracking-Beam
arrays and Switching-Beam Arrays Operating in a CDMA Mobile
Communication Channer", IEEE Antennas and Propagation Magazine,
December 1999, and "Smart Antennas", IEEE Antennas and Propagation
Magazine, June 2000, "Smart Antennas for Mobile Communication
systems: Benefits and Challenges", Electronics and Communication
Engineering Journal, April 1999, it has been shown that
adaptive-beam systems perform better than switched-beam systems
especially in high interference environments. They perform better
partly because they take spatial and temporal correlations of
interfering signals into account and eliminate the need for
frequent handovers within a sector and also tend to maintain
maximum antenna gain in the desired direction Adaptive-beam systems
can be used to track individual mobile terminals within a base
station service area. Several different methods are disclosed in
the literature that all attempt to find an array weighting vector
that maximizes the SINR for a desired signal. These methods vary in
complexity. Estimation of signal parameters by rotational
invariance techniques (ESPRIT) is more widely used than the other
sub-space eigen-decomposition method Multiple Signal Classification
(MUSIC). Although MUSIC is considered to achieve higher resolution
it also requires more computation in its searching algorithm than
the closed form solution provided by ESPRIT. These
eigen-decomposition methods require a good estimate of the array
covariance matrix by averaging over time, such that in the limit
where the averaging time approaches infinity the estimate becomes
exact. The array covariance matrix is found by averaging over
several snapshots of the array signal values. Once determined the
matrix can be updated after every sample. Several recent patent
disclosures cite the use of ESPRIT for adaptive beam-forming such
as U.S. Pat. No. 6,008,759 entitled "Method of Determining the
Direction of Arrival of a radio Signal, as well as Radio Base
Station and Radio Communications Systems" and U.S. Pat. No.
5,892,700 "Method for the High Resolution Evaluation of Signals for
One or Two Dimensional Directional or Frequency Estimation". In the
preferred embodiment of the former, a sub-optimal method is
introduced that forms a beam based on the steering vector
determined from the strongest eigen-value. This eliminates the need
for full eigen-decomposition and significantly reduces computation
time allowing faster track updates. It is considered sub-optimal
because it does not attempt to place nulls on significant
interference and thus does not maximizing SINR. However, it is
suggested that interference may be suppressed further by standard
side-lobe control methods. Furthermore, ubiquitous multi-path
propagation with uncorrelated fading would require at least two
paths to be resolved requiring means for several channels. In one
embodiment of the latter, ESPRIT is used to resolve several
multi-path signals from a single desired source simultaneously and
therefore take advantage of maximal ratio combining. Although this
technique is considered optimum combining because it maximizes SINR
it has the disadvantage that its solution does not necessarily
place the peak of an antenna beam on the desired signal path. The
effect of this is the degradation in sensitivity of the system to
the thermal noise thus reducing the range of the base station.
[0006] The methods described in the above-mentioned references
require computationally expensive eigen-decomposition of the
estimated array covariance matrix requiring at least an (M.times.M)
matrix inversion where M is the number of antenna elements.
Accordingly as the number of users, K, and the number of elements,
M, grows the matrix manipulation will become unwieldy and memory
intensive. Conversely, the method and apparatus of this invention
replaces the (M.times.M) matrix inversion to a single computation
of a ratio. Furthermore, the above-mentioned references describe
methods that incorporate switched and fixed beam solutions that
carry the burden of frequent handovers.
[0007] Other less computationally intensive methods for adaptive
beam-forming and direction finding do exist. For example, a simpler
means of DOA estimation disclosed in U.S. Pat. No. 6,212,406,
"Method for Providing Angular Diversity, and Base Station
Equipment", outlines a search and track by scan method, relying on
beam-width modulation, determines directions and delays of signals
by seeking the strongest power levels or largest SINR. In a
multi-path environment where the signal could jump discontinuously,
too much time could elapse before reacquiring the signal.
Furthermore, searching for a maximum signal to determine whether
maximum antenna gain has been achieved will prove difficult for
near-in high speed mobile units and signals experiencing fast
fading. Signal level measurement uncertainty could also be
construed as beam-width modulation further degrading accuracy.
[0008] Extensively used in many Radar and Sonar discriminators,
another successful technique utilized for DOA tracking is known as
mono-pulse beam forming and is described in "Introduction to Radar
Systems", M. I. Skolnik, 1980. Unlike ESPRIT and MUSIC techniques,
mono-pulse estimation of DOA requires only the determination of a
single ratio of two signals. The two signals are generated by
forming two different beams from a single antenna: 1) a summation
beam, 801, containing the signal information that is ultimately
carried through the rest of the network and 2) a difference beam,
802. Over angle space this ratio is a well behaved function, 901
from which an accurate estimate of DOA relative to the current beam
position may be determined. Its accuracy is an improvement over
beam peak finding because of the sharpness of the difference beam
null relative to the broad nature of the antenna beam. Its
performance does rely on the ability to find the zero of the
difference beam null and in a high interference or noisy
environment this null tends to fill increasing the uncertainty of
the angle offset estimate. Therefore, this technique requires a low
interference environment. However, mobile communication lends
itself to this technique due to the separation of radio links via
various multi-access schemes such as CDMA TDMA, and FDMA. Thus, low
interference is achieved through the orthogonality of the
co-channel users.
[0009] Historically, mono-pulse tracking, although simple to
implement, has not been utilized in multiple access communication
systems. Digital beam-forming has only recently started to make a
presence in practical systems due to the growth in processing
speeds. Prior to digital implementations beam-forming systems have
typically been realized in analog. To realize multiple beams in
multiple access systems would require separate analog channels in
the antenna beam-formers, including separate phase shifters and
attenuators. The number of phase shifters and attenuators could
number in the hundreds and even thousands per antenna, depending on
the capacity of the system and the number of antenna elements in
the phased array. This limits the number of simultaneous multiple
beams to tens-of-beams and not hundreds-of-beams required for
multi-access communication. Mono-pulse tracking has not been
previously implemented for this application because it implies the
real-time tracking of multiple simultaneous beams. Digital
processors and ASIC's have just recently surpassed the performance
requirements to achieve such a result. However, computational
resources are still and always will be considered premium. Thus, a
need exists to preserve as much of the computational resources as
possible while enabling a significant increase in the capacity and
range of communication systems.
SUMMARY OF THE INVENTION
[0010] In one embodiment of the invention the capacity and range of
mobile or fixed wireless communication base stations are improved
by implementing a single or multiple antenna beam per signal path.
Adaptive beam-forming based on up-link direction-of-arrival
estimation can be performed without using the above-mentioned
computationally intensive techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates the low resolution angle estimation with
a set multiple fixed beams in a multi-user multi-path
environment.
[0012] FIG. 2 depicts a snapshot of a set of agile beams prior to
track.
[0013] FIG. 3 depicts a snapshot of a set of agile beams positioned
at locations determined by the fixed beam search processor.
[0014] FIG. 4 depicts a snapshot of a set of agile beams locked
onto their respective signal paths.
[0015] FIG. 5 depicts a block diagram of the fixed beam search
processor according to an embodiment of the invention.
[0016] FIG. 6 depicts a block diagram of the agile beam search
processor according to an embodiment of the invention.
[0017] FIG. 7 depicts a block diagram of an agile beam element
according to an embodiment of the invention.
[0018] FIG. 8 illustrates the relation of a .SIGMA.-beam pattern
with a .DELTA.-beam pattern.
[0019] FIG. 9 illustrates the mono-pulse angle estimation curve
according to an embodiment of the invention.
DETAILED DESCRIPTION
[0020] Adaptive beam-forming can be implemented digitally in the
base-band. Therefore, some means of down-converting an RF signal to
base-band as well as a scheme for calibration is assumed. The
algorithm can be divided into two steps: 1) a low resolution search
performed by the fixed beam searcher, 2) high resolution tracking
which utilizes standard mono-pulse techniques. Alternatively, if
the approximate location of the mobile or fixed wireless user is
known or is determined by other means the low resolution track can
be circumvented with high resolution mono-pulse tracking only. The
simplicity of this direction-of-arrival estimation calculation will
enable real time tracking of a mobile station within the sector
area serviced by the antenna.
[0021] The low resolution search is performed by comparing the
signals in each of the beams that sub-divide the sector, (eg.
101-108). The signals may be line of sight (LOS) or a time delayed
non-LOS multi-path. The base station equipment will decide which
and how many of the signals it will need, to meet the required
signal-plus-interference-to-noise ratio (SINR) for that service.
The base station will then cue multiple agile and autonomous high
resolution tracking beams to within +/-1/2 beam-width where each of
these signals reside. The agile beam is initially formed by phasing
the signals of each element in the array with a set of weighting
vectors corresponding to that fixed beam (101-108). With the agile
beam cued to the approximate location, the mono-pulse estimator
will determine the fine adjustment needed to lock on the signal.
The agile beam becomes autonomous and tracks the signal wherever it
is within the 120.degree. sector until it is handed off to another
sector or base station or the signal is lost. In a multi-path
environment with many obstructions, a signal from a mobile station
will likely become abruptly shadowed. Under such circumstances, the
tracking algorithm will lose the signal and its track. However, an
embodiment of this invention is the implementation of a low
resolution fixed beam search that can be running continuously in
the background, so that within the next timeslot or symbol another
signal will be acquired if one exists within the field-of-view of
the antenna.
[0022] Mono-pulse DOA is performed by measuring the relative signal
levels of two different beams formed with the same antenna. The
summation beam is formed by the summation of coherently phased
signals from a plurality of elements within an antenna array. The
difference beam is formed by symmetrically dividing the antenna and
summing the signals from one half of the elements, exactly out of
phase with the remaining half of the elements. The ratio of the
difference beam signal relative to the summation beam signal
results in a signal proportional to the magnitude of the angular
offset and whose polarity indicates to which side of the beam peak,
the signal is present. The tracking loop then tends to keep the
maximum of the antenna gain in the desired direction by keeping the
signal centered on the zero of the corresponding difference
pattern.
[0023] For code-division multiple access (CDMA) systems it is
assumed that the RF signal has been matched filtered and
de-modulated to a digital base-band signal prior to any
beam-forming. This assumption includes the low resolution fixed
beam search as well as for every agile beam. Mono-pulse estimation
therefore can be performed after de-scrambling and de-spreading
separates the desired signals from other co-channel interference
(CCI). This ensures that tracking accuracy and precision is not
degraded. For time-division multiple access (TDMA) systems it is
assumed that the radio-frequency (RF) signal has also been matched
filtered and delayed and that synchronization is established. The
beam-formed signal can then be directly applied to the mono-pulse
estimation circuitry.
[0024] Downlink beam-forming can be achieved by utilizing the DOA
parameters derived on the uplink. In FDD systems where the
separation of transmit and receive frequency bands are relatively
narrow it is known that DOA parameters are virtually invariant to
frequency. To reduce the probability that a single downlink signal
will undergo a deep fade, two or more signals can be transmitted
through multiple downlink beams derived from uplink DOA estimation.
The likelihood that the mobile will encounter a deep fade in two
different multi-paths is low. Therefore, it would be prudent to
transmit at least two signals in two downlink beams as disclosed in
U.S. patent application "Method and Apparatus for Received
Uplink-Signal Based Adaptive Downlink Diversity Within a
Communication System" [Attorney Docket No. EVOY002/00US]. The
uplink DOA information provided by this search and track method can
be directly applied to the downlink with the appropriate frequency
transformation of the array weighting vectors.
[0025] Angle estimation in one plane with an M-element linear array
has been considered above. This approach can be extended to 2-D DOA
estimation in both the azimuth and elevation planes utilizing a
2-dimensional (M.times.L)-element planar array symmetrically
divided into left-right and top-bottom halves. The signal from each
element is used to construct, not two beams as with the linear
array discussed above but a cluster of three beams that are slaved
to each other. The summation beam is formed by taking the sum of
all of the signals from each of the elements after they have been
phased properly to correspond with an appropriate steer angle which
lies within a specified maximum conical angle relative to the
normal of the plane of the array. The azimuth difference beam is
formed by summing the signals from every element after the same
phase gradient for summation beam has been applied to all of the
elements with an additional 180.degree. phase shift applied to the
elements comprising the left half of the array relative to the
elements comprising the right half of the array. The elevation
difference beam is formed by summing the signals from every element
after the same phase gradient for summation beam has been applied
to all of the elements with an additional 180.degree. phase shift
applied to the elements comprising the top half of the array
relative to the elements comprising the bottom half of the array.
This technique, when coupled with some means of time-of-arrival
estimation enables location based services.
[0026] The theoretical gains achieved by any smart antenna assume a
uniform distribution of users across a sector. If the density is
non-uniform, the higher density sub-sectors will suffer a
degradation in capacity improvement. However, multi-user detection
(MUD) schemes can also be integrated with the digital beamformer to
recover some of that loss. For example, if there are a cluster of
users within a small angular space it might be prudent to use one
or more of the fixed beams and some form of interference
cancellation normally associated with (MUD) to improve the
SINR.
[0027] It has been accepted that smart antennas offer an additional
degree of freedom for operators to improve the capacity and range
of their systems. The methods of this invention are much less
complex, and thus less costly than many of the other
computationally intensive eigen-decomposition techniques such as
ESPRIT and MUSIC. It will also perform far better than existing
sectorization methods by achieving a high gain antenna beam on
every user while simultaneously eliminating the need for frequent
handovers. Furthermore, the mono-pulse technique has been a proven
method over several decades in single beam Radar systems. Digital
processing technologies have now enabled such a technique to be
used in the multi-access, multi-user, fixed and mobile wireless
communication systems.
[0028] The adaptive beam-forming method comprises two functions: 1)
low resolution angle estimation performed by the fixed beam
searcher and 2) high resolution angle estimation performed by the
agile beam tracker.
[0029] FIG. 1 illustrates a multi-user multi-path path environment
illuminated by a set of fixed beams that sub-divide the sector.
Fixed beam searching is used for low resolution tracking and the
initial acquisition of a user. The fixed beam search (FBS)
processor segments the entire (120.degree.) sector 116 into M beams
(eg. 101-108), with cross-over points at a predetermined level X dB
118, usually 3 dB corresponding to the 3 dB beam-width of the
antenna.
[0030] Signals arriving at the base station antenna may arrive as
line-of-sight (LOS) or multi-path. Any one of the beams will
contain the signals from users within the main lobe of the beam as
well as attenuated versions of signals from users, outside of the
main beam. The attenuation levels are a function of the sidelobe
levels of the antenna pattern, 120,121,122,123. As shown in FIG. 1,
there are two users 113,114 that are within the field-of-view (FOV)
of the base station antenna 119. The signal 117 arriving at the
base station from 113 is a LOS signal incident from the direction
that beam 101 covers. The FBS will therefore assign that signal to
sector 101. The is no direct LOS path from 114 to the base station
antenna, however, two multi-path signals 109, 110 are present and
incident from directions covered by beams 108, 103, respectively.
The FBS will therefore assign the two signals to sectors 108 and
103.
[0031] Turning to FIG. 5, the M beams 101-108 are formed in the
Fixed Beam Searcher (FBS) 502 by performing a spatial transform of
N complex samples taken from each of the N elements at M (in this
example M=8) discrete points in angle space. Therefore, 1 F ( m ) =
n = 1 N C ( n ) W n , m 1
[0032] where,
[0033] F(m) is complex and contains the total signal in beam m,
[0034] C(n) is the complex spatial sample from element n,
[0035] W.sub.n,m is the complex weight applied to element n for
beam m,
[0036] n is the number of the element in the array from 1 . . .
N,
[0037] m is the number of the beam from 1 . . . M.
[0038] The complex signal F(m) 505 from the m.sup.th beam is then
de-scrambled, de-spread or de-interleaved in the User Processing
Element (UPE) 506 for every user or multi-path signal k=1 . . . K.
The UPE functions as the correlation receiver and separates users
from one another by de-spreading CDMA signals or de-interleaving
TDMA signals. The UPE does not necessarily have to decode the
polarity of the bit and therefore no channel phase estimation is
required. It is sufficient only to determine the magnitude of the
bit. A detector 507 for each user then determines the beam(s) with
the maximum signal level and integration may be performed to
increase the signal-to-noise prior to making the decision.
[0039] In the example, the FBS locates the signal from user 113 and
assigns an agile beam to it by passing the associated (N.times.1)
complex weight vector, {overscore (m)}.sub.117.sup.fixed, 508
corresponding directly with beam 101 to the agile beam search
processor. Also in the example, the FBS locates the signal
direction of both multi-path signals from user 114 and assigns two
agile beams to them by passing the associated set of compex
weights, {overscore (m)}.sub.109.sup.fixed and {overscore
(m)}.sub.110.sup.fixed corresponding directly with beams 108 and
103, respectively, to the agile beam search processor.
[0040] The agile beam search processor contains at least K, agile
beam elements, (ABE), 602 (shown in FIG. 6). Initially, in each
ABE, a comparison is made between the current weights
W.sup.i.sub.n,k and the weights associated with the beam where the
signal is located, 712. If the direction associated with the weight
W.sup.i.sub.n,k is within one beam-width of that found by the FBS
than the agile beam is tracking the signal and the beam becomes
autonomous, otherwise W.sup.i.sub.n,k is reset to {overscore
(m)}.sub.k.sup.fixed.
[0041] FIGS. 2-4 represent snapshots of beam positions as the agile
beam processor tracks and locks onto the several signals from the
several users. In the example, three agile beam elements, 602, are
assigned to two users. One user, 113 is assigned one ABE and the
other user 114 is assigned two ABE's. Prior to any information
about the DOA of each signal, the beam positions are arbitrary 201,
204, 207. Within the next symbol, slot, timeslot, or frame the beam
positions are cued to the established sectors 301, 304, 307
previously determined by the FBS. With the agile beams cued to the
correct sectors they become active and begin high resolution track
utilizing mono-pulse estimation.
[0042] FIG. 8 illustrates that a mono-pulse tracking system is
implemented by forming two beam patterns shown: the sum pattern 801
(shown solid with a peak at 0.degree.) and the difference pattern
802 (shown dashed with a null at 0.degree.). Both of these patterns
are formed using the single antenna or aperture. The sum pattern is
used to provide absolute power information of the received signal
and is simply formed by summing the signals from several elements
of an array. The difference beam is formed by partitioning the
array into two halves phasing one half of the aperture to
180.degree. with respect to the other and then summing the two
signals from both halves. The ratio of the complex voltages
provided by the sum and difference beams provides the
discrimination information for DOA estimation and beam
tracking.
[0043] Referring to FIG. 9, it is seen that computing the real part
of the complex ratio of the difference pattern with respect to the
sum pattern over angle space, 2 ( ) = RE [ ( ) ( ) ] 2
[0044] results in the mono-pulse angle estimation curve of 901.
This curve, representing the ratio of complex voltages at the sum
and difference beam signal ports, is proportional to the DOA of the
incident energy relative to the current beam position. For example
if the user is located at 0.degree., .DELTA.(.theta.)=0 and thus
.psi.(.theta.)=0, depicting that tracking can be implemented with a
feedback loop that attempts to keep the beam centered at the zero
of the difference pattern.
[0045] FIG. 7 depicts a block diagram of the mono-pulse beam-former
and the tracking loop. Each ABE processes signals from a plurality
of antenna elements, N. The sum beam is formed in the .SIGMA.
beam-former 701 by performing the spatial transform as follows 3 (
k ) = n = 1 N C ( n ) W n , k 3
[0046] where,
[0047] .SIGMA.(k) is complex and contains the total signal in beam
k,
[0048] C(n) is the complex spatial sample from element n,
[0049] W.sub.n,k is the complex weight applied to element n for
beam k,
[0050] n is the number of the element in the array from 1 . . .
N,
[0051] k is the number of the user or the user's signal.
[0052] The difference beam is formed in the A beam-former 702 by
performing the spatial transform as follows 4 ( k ) = n = 1 N C ( n
) D n , k 4
[0053] where,
[0054] .DELTA.(k) is complex and contains the total signal in beam
k,
[0055] C(n) is the complex spatial sample from element n,
[0056] D.sub.n,k is the complex weight applied to element n for
beam k,
[0057] n is the number of the element in the array from 1 . . .
N,
[0058] k is the number of the user or the user's signal.
[0059] The .SIGMA. beam signal for a specified user, 703, is
de-scrambled, de-spread or de-interleaved in the .SIGMA. Channel
Estimator 707. Utilizing a pilot bit, the .SIGMA. channel estimator
may also be used to estimate signal phase, .phi. from the control
channel for W-CDMA systems. It can also deliver an integrated
.SIGMA. signal with higher SNR to the mono-pulse estimator. In
W-CDMA systems the handset modulator adjusts the gain of the
control channel relative to the traffic channel depending on the
data rate. Higher data rates require more signal energy to be
stripped from the control channel in order to maintain a specified
bit-error-rate (BER). To accurately estimate the channel phase,
this integration may be performed to raise the signal-to-noise
ratio of the control channel. The .DELTA.-demodulator, 706 performs
the same de-spreading, de-scrambling or de-interleaving and
integration as the .SIGMA.-channel estimator. However, if a phase
estimate is needed it will be provided by the .SIGMA.-channel
estimator.
[0060] The angle estimation is performed by first taking the ratio
of signals from the sum and difference channels, as in Eq 1, 708a.
Then, by converting the dependent variable .psi. into an
independent variable and by converting the independent variable
.theta. into a dependent variable u=sin(.theta.), an expression can
be formed for the angle error relative to the difference beam
null,
.delta.u(.psi.)=f(.psi.) 5
[0061] The function .delta.u(.psi.) is odd and can be approximated
by a Taylor polynomial,
.delta.u(.psi.)=a.sub.0+a.sub.1.multidot.(.psi.)+a.sub.3.multidot.(.psi.).-
sup.3 6
[0062] where, .delta.u, is the angle prediction in sine space, 5 =
RE [ ] .
[0063] Antenna patterns are a function of several variables
including aperture weighting, mutual coupling, array design, and
beam position. Thus, the mono-pulse estimation coefficients
including, a.sub.i will also be dependent on the same parameters.
These parameters are gathered by simulation and by measurement.
Once gathered they are stored in a look-up table 709 and retrieved.
A beam position correction can then be calculated, by using Eq 6,
and mapping it to a phase correction
.DELTA.W.sub.n,k.sup.i+1 7
[0064] for all of the elements, 710. This phase correction is then
applied to the current agile beam position, by recalculating the
phase weights
W.sub.n,k.sup.i+1=W.sub.n,k.sup.i.multidot..DELTA.W.sub.n,k.sup.i+1
8
D.sub.n,k.sup.i+1=D.sub.n,k.sup.i.multidot..DELTA.W.sub.n,k.sup.i+1
9
[0065] 711, once it has been verified, 712, that the signal is
still in the same sector.
[0066] In the example the multi-path signal, 109 of FIG. 3 and FIG.
9 is shown to be left of beam center. The ratio, .psi. is then
calculated using Eq. 2, 708a the angle error is estimated using Eq.
6, 708b, which is mapped to a phase correction, 710 and then
applied to the current beam position using Eq. 8 and 9, 711. The
result is a new beam, 401 that is now centered on the signal.
[0067] Conclusion
[0068] A first method positions the highest antenna gain on the
multiple signal paths of a communication link between two or more
communication devices within a sector. The signal within the sector
is acquired and its location within the portion of the sector is
determined by utilizing a low resolution search comprising of a set
of fixed antenna beams that divide the associated sector into its
portions. By evaluating the optimal signal of those provided by
each fixed beam, the location is ascertained by mapping the beam
position to a portion of the sector. This mapping also produces a
set of antenna array weighting vectors for each and every
communication link within the sector of the communication system
associated with the antenna.
[0069] A secondary set of antenna beams is provided for each and
every desired signal path from the multiple users. The initial set
of antenna array weighting vectors are determined by the low
resolution search. This set of agile beams can be scanned
continuously within the sector while tracking the signals from the
users within the sector. Once initial acquisition is achieved agile
beam tracking is performed by mapping an angle offset determined
from the ratio of two signals. These signals are provided by two
slaved beams, the summation beam and the difference beam, that
comprise a single agile beam. The relationship of Equation 2
determines an amplitude that can be mapped as an angle offset
relative to the null of the difference beam using Equation 6. The
polarity of the result of Equation 2 determines which side of the
null the signal resides. Based on this offset a new antenna
weighting vector is calculated by equation 7, 8,and 9 and applied
to antenna elements. This continues at a rate consistent with a
timeslot (TDMA) or symbol (CDMA) until the user is handed of to
another cell or sector within the cell.
[0070] A second method positions the highest antenna gain on the
signal path of a communication link between two or more
communication devices within a sector. A primary set of antenna
beams is provided for each and every desired signal path from the
multiple users. The initial set of antenna array weighting vectors
are predetermined from a known user signal path location such as
with fixed wireless systems or from feedback information provided
by a mobile user about its approximate location within the sector
or cell. A set of agile beams can be allocated to each user and
scanned continuously within the sector while tracking the signals
from the users within the sector. Once initial acquisition is
achieved agile beam tracking is performed by mapping an angle
offset determined from the ratio of two signals. These signals are
provided by two slaved beams, the summation beam and the difference
beam, that comprise a single agile beam. The relationship of
Equation 2 determines an amplitude that can be mapped as an angle
offset relative to the null of the difference beam using Equation
6. The polarity of the result of Equation 2 determines which side
of the null the signal resides. Based on this offset a new antenna
weighting vector is calculated by equation 7, 8,and 9 and applied
to antenna elements. This continues at a rate consistent with a
timeslot (TDMA) or symbol (CDMA) until the user is handed of to
another cell or sector within the cell.
* * * * *